feat: create platform utility using Tauri OS plugin

src/lib/utils/platform.ts

@@ -0,0 +1,23 @@
+/**
+ * Platform detection via @tauri-apps/plugin-os.
+ *
+ * `platform()` is synchronous (compile-time constant baked into the binary).
+ * Falls back to 'linux' in non-Tauri environments (vitest, SSR, svelte-check).
+ *
+ * NOTE: The `macos` CSS class on `<html>` is set separately by an inline
+ * script in app.html for FOUC prevention — that is intentionally kept.
+ */
+
+import { platform as tauriPlatform } from '@tauri-apps/plugin-os';
+
+type Platform = 'macos' | 'linux';
+
+let detectedPlatform: Platform;
+try {
+  detectedPlatform = tauriPlatform() as Platform;
+} catch {
+  // Fallback for non-Tauri environments (vitest, SSR, svelte-check)
+  detectedPlatform = 'linux';
+}
+
+export const platform: Platform = detectedPlatform;